SIGNIFICANCE AND EARLY IDEAS 7 



carried out ringing experiments and observed increased 

 swelling above rings and between rings if leaves were 

 present, but no swelling if they were absent. He definitely 

 states, however, that this increased growth is not due to a 

 stoppage of the sap in its return downward. He demon- 

 strated the possibility of a backward flow of water through 

 the wood and suggested a flow and ebb of sap through 

 the wood. Knight (1801) carried out a number of ringing 

 experiments which showed that tissues below rings failed 

 to grow unless a shoot or leaves were present. Parts of the 

 stem isolated from the rest of the stem by rings showed 

 growth in diameter if leaves were present but no growth if 

 leaves were absent from the isolated portion. Knight 

 found that colored solutions would readily move through 

 the woody part of the stem but not through any tissue 

 external to the wood. On examination of the bundles of 

 tubes leading through the petioles of apple and horse- 

 chestnut leaves, only some of them were found to be 

 stained with these colored solutions. He traced the 

 uncolored tubes backward and found them to lead through 

 the petiole to the inner bark and not to the wood. He 

 suggested that these were the tubes concerned in the back- 

 ward movement of materials prepared by the leaves. 

 Removal of leaves or a reduction of their area correspond- 

 ingly reduced the growth of the tissue below their insertion. 

 From experiments which showed that the removal of the 

 pith — which also was not stained by the colored sap — did 

 not hinder transfer, he concluded that it is not concerned in 

 backward translocation. 



Since these early observations many detailed studies have 

 been made on the occurrence, structure, and arrangement 

 of these tissues which are supposedly the principal channels 

 for the backward translocation of foods. These tissues 

 commonly consisting of several types of cells are usually 

 spoken of as phloem tissues, although ''bast" and ''bark" 

 are occasionally used as synonymous terms. 



The large number of ringing experiments that have been 

 carried out since the time of Malpighi seem to have proved 



